The present invention relates to PEM fuel cells, and more particularly to a composite separator plate having conductive fibers extending therethrough to enhance electrical conductivity and a method for manufacturing the same.
Fuel cells are being developed as a power source for many applications including vehicular applications. One such fuel cell is the proton exchange membrane or PEM fuel cell. PEM fuel cells are well known in the art and include in each cell thereof a membrane electrode assembly or MEA. The MEA is a thin, proton-conductive, polymeric, membrane-electrolyte having an anode electrode face formed on one side thereof and a cathode electrode face formed on the opposite side thereof. In general, the membrane-electrolyte is made from ion exchange resins, and typically comprise a perfluoronated sulfonic acid polymer such as NAFION(trademark) available from the E.I. DuPont de Nemeours and Co. The anode and cathode faces, on the other hand, typically comprise finely divided carbon particles, very finely divided catalytic particles supported on the internal and external surfaces of the carbon particles, and proton conductive particles such as NAFION(trademark) intermingled with the catalytic and carbon particles; or catalytic particles, without carbon, dispersed throughout a polytetrafluorethylene (PTFE) binder.
The MEA is interdisposed between sheets of porous, gas-permeable, conductive material which press against the anode and cathode faces of the MEA and serve as the primary current collectors for the fuel cell, and the mechanical support for the MEA. Suitable such primary current collector sheets comprise carbon or graphite paper or cloth, fine mesh, noble metal screen, and the like, as is well known in the art. This assembly is referred to as the MEA/primary current collector assembly herein.
The MEA/primary current collector assembly is pressed between a pair of non-porous, electrically conductive separator plates which serve as secondary current collectors for conducting current between adjacent fuel cells internally of the stack (i.e. in the case of bipolar plates) and at the ends of a cell externally of the stack (i.e. in the case of monopolar or end plate). The separator plate contains a flow field that distributes the gaseous reactants (e.g. H2 and O2/air) over the surfaces of the anode and the cathode. These flow fields generally include a plurality of lands which contact the primary current collector and define therebetween a plurality of flow channels through which the gaseous reactants flow between a supply header and an exhaust header located at opposite ends of the flow channels.
Conventionally, a separator plate is formed of a suitable metal alloy such as stainless steel or aluminum protected with a corrosion resistant, conductive coating for enhancing the transfer of thermal and electrical energy. Such metal plates require two stamping or etching processes to form the flow fields and either a bonding or brazing process to fabricate a cooled plate assembly which adds cost and complexity to the design. In addition, the durability of the metal plate in the corrosive fuel cell environment and the possibility of coolant leakage remains a concern.
These drawbacks have led to the development of composite separator plates. In this regard, recent efforts in development of a composite separator plate have been directed to materials having adequate electrical and thermal conductivity. Material suppliers have developed high carbon loading composite plates consisting of graphite powder in the range of 70% to 90% by volume in a polymer matrix to achieve the requisite conductivity targets. Separator plates of this type survive the corrosive fuel cell environment and, for the most part, meet cost and conductivity targets. However, due to the high graphite loading and the high specific gravity of graphite, these plates are inherently brittle and dense which yield less than desirable volumetric and gravimetric stack power densities.
Additionally, efforts have been made to reduce the fuel cell stack mass and volume by using thinner plates. Unfortunately, the brittle nature of these plates frequently results in cracking and breaking, especially in the manifold sections of the plate, during part demolding, during adhesive bonding, and during stack assembly operations. As such, a separator plate having a relatively low carbon concentration and relatively high-polymer concentration is desirable to reduce the brittleness of the separator plate and to meet fuel cell stack mass and volume targets. Unfortunately, at low carbon concentrations, it is extremely difficult to meet the desired electrical and thermal conductivity targets.
Fibrous materials are typically ten to one thousand times more conductive in the axial direction as compared to conductive powders. Consequently, a polymeric separator plate having a conductive fibrous material disposed therein would increase the electrical conductivity of the plate without having a relatively high concentration of carbon loading which may lead to brittleness. However, to achieve these benefits, the fibrous materials must be properly oriented in a through plane direction. Furthermore, a polymeric separator plate having a conductive fibrous members extending continuously therethrough in a through plane orientation would greatly enhance the transfer of electrical energy through the separator plate.
Thus, there is a need to provide a fuel cell separator plate and a method of manufacture which overcomes the inherent problems associated with high carbon loaded plates and the difficulties associated therewith. Therefore, it is desirable to provide a fuel cell separator plate formed of a robust material having a conductive fibrous material extending therethrough to enhance the electrical conductivity of the separator plate. It is also desirable to provide a fuel cell separator plate having integrally formed cooling channels to reduce the thermal energy in the plate and the possibility of coolant leaks in the separator plates. It is further desirable to provide a method of manufacturing such fuel cell separator plates which reduces the number of steps in fabricating cooled plates (i.e. eliminate double forming and bonding).
The present invention provides a composite separator plate for use in a fuel cell stack. The composite separator plate includes a plurality of elongated support members oriented generally parallel to each other and a polymeric body portion formed around the support members. The body portion has a first surface with a plurality of flow channels and a second surface opposite the first surface. A plurality of electrically conductive fibers are disposed within the polymeric body portion, each fiber extending continuously from the first surface of the polymeric body portion to the second surface of the polymeric body portion in a through plane configuration.
The present invention also provide a method of manufacturing a composite separator plate wherein a plurality of elongated support members are oriented in a generally parallel arrangement. A plurality of conductive continuous fibers are arranged adjacent the plurality of support members to form a lattice. The lattice is encased in a polymeric material to form a non-porous separator plate having a first and second surface. A plurality of channels are formed in at least one of these surfaces. A portion of the first and second surfaces are removed to form exposed surfaces and to sever the continuous fibers into a plurality of conductive elements having ends which terminate at the exposed surfaces. Each of the plurality of conductive elements extend continuously in a through plane direction through the separator plate.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.